The roughness of the bone matrix surface affects osteoblastic differentiation. However, the effect of the roughness of the matrix surface on osteoclastic bone resorption remains to be studied. We examined the latter effect using disaggregated osteoclasts from neonatal rats. Slices of bovine cortical bone, plates of titaium and hydroxyapatite were used as substrates for osteoclastic resorption. Although the number of osteoclasts on each substrate decreased with incubation period, the number of osteoblasts increased on the rough surface. The development of osteoclasts on the titanium plates was lower than other substrate, and it seemed to be based on physical or chemical factor of the substrate. In bone slices and hydroxyapatite plates, the resorption pit number and the total pit area on the rough surface were not different from those on smooth surfaces after 1 day, but they were 2 or more times higher after 3 days. The alkaline phosphatase (ALP)-positive osteoblasts were relatively rare in both types of slices at first, then the number and the diameter of the enzyme-positive cells and the clusters preferentially increased on the rough bone slices. When hydroxyurea was added to the culture in order to suppress the proliferation and the subsequent differentiation of osteoblastic cells on rough surfaces, the increase in resorption on the rough surfaces was effaced ; however, this agent had little affect on resorption of the smooth surfaces. The addition of ALP-positive cells to disaggregated osteoclasts increased bone resorption on the smooth surface. The results suggest that osteoblast development and subsequently bone resorption by osteoclasts is enhanced by the roughness of matrix surfaces.